The invention relates to liquid hydrocarbons such as hydrocarbon fuels, hydraulic fluids, lubricants or solvents, comprising cyclic ortho ester dehydrating icing inhibitors. The hydrocarbon fuels include aviation fuels. Methods of inhibiting ice crystal formation and ice inhibiting compounds are also provided.
Solubility of water in the current context refers to the maximum water concentration which can dissolve in any given amount of hydrocarbons at a specific temperature and air humidity; this is strongly dependent upon the chemical composition of the fuel itself [1, 2].
Dissolved water is often a normal component of liquid hydrocarbons and is vaporised on combustion of hydrocarbon fuels. However, free water can freeze and block fuel or other feed lines. The water can also support microbial growth and contribute to corrosion. There is therefore a need to remove water or reduce the amount of ice crystal formation in liquid hydrocarbons.
There is a need to identify kinetically fast, lipophilic water scavengers that can produce on hydrolysis hydrophilic ice inhibitors.
Aviation turbine fuels are largely straight-run distillates of crude oil and their composition comprises, by volume, between 99%−99.5% hydrocarbons, with the remaining fraction of heterorganic components containing the elements S, O and N. Jet fuel is a mixture of many different hydrocarbons; most of them can be grouped into three broad classes: paraffins (or alkanes), naphthenes (or cycloalkartes) and aromatics (or alkylbenzenes) [1, 3, 4]. The proportions of these three classes varies depending upon the source of the crude oil from which the fuel is derived and the refining process. The aromatic content is limited to 25% v/v [5]. The formation of attractive electrostatic interactions between water and π-aromatic systems would account for the enhanced solubility of water in alkylbenzenes relative to both paraffins and naphthenes [4].
The aviation industry uses both hardware and quality control procedures to protect jet fuel from water contamination [1]. However, even if jet fuel enters the fuel tank without free water, this does not prevent its formation.
Indeed, dissolved water present in fuel will precipitate out of solution in the form of micro droplets as the fuel temperature drops through diurnal cycles or during operation as the aircraft climbs to altitude [6, 7]. Water solubility in the fuel decreases by approximately 2 ppm (volume) per 1° C. For example, a temperature change from 20→−10° C. creates ca. 29 ppm (volume) of dissolved water to be liberated as free water, which in 100 tons of fuel amounts to 3.62 L of free water [1].
A second source of free water in fuel tanks derives from the condensation of atmospheric moisture. As fuel is consumed, air is drawn into the fuel tanks through the vent system; the moisture in the air condenses as it comes in contact with the cold fuel and tank surfaces at the end of a long flight [8]. Free water can starve engines, support microbial growth, contribute towards corrosion and furthermore freeze, risking plugging filters in the aircraft. The industry has developed equipment and procedures to mitigate these problems, but challenges remain [1, 3].
The aviation industry uses a variety of additives to counter the detrimental effects of free water. Biocides are added to prevent microbial growth, corrosion inhibitors are used to protect uncoated steel tanks and pipelines from corrosion and improve the lubricity of fuels, and Fuel System Icing Inhibitors (FSII) are added to inhibit ice formation.
Since ethylene glycol monomethyl ether (EGME) was banned because of its toxicity to humans and the environment, the only FSII currently approved for Jet A, Jet A-1, and military fuels is di-ethylene glycol monomethylether (di-EGME) [1, 5]. FSII are hydrophilic substances, dissolving in any free water that forms, disrupting hydrogen-bonding networks responsible for molecular ordering—thereby lowering the freezing point of the liquid by preventing crystallisation [3]. The concentration of di-EGME must be in the range 0.10-0.15% v/v [1,5]. At these concentrations, Trohalaki et al. found that water (0.007% v/v) in jet fuel freezes below −36° C. [9]. Di-EGME has also proved to be an effective deterrent to microbial growth [1].
The aviation industry has identified several problems related to the interaction of FSII and “wet” fuel, which can actually undermine fuel protection.                In addition to being hydrophilic, di-EGME is also hygroscopic—this additional characteristic leads to an uptake of atmospheric water during blending operations which affects its solubility in fuel.        Di-EGME has a very high water-fuel partitioning ratio: it preferentially dissolves in water. This leads to denser water layers separating under gravity with up to 40-50 wt % of di-EGME. Unless replenished, the jet-fuel is essentially “stripped” of its protection against icing.        The presence of di-EGME in the water contained within the fuel alters interfacial properties, and can impair the performance of filters and monitors. Particularly, the separation efficiency of filter/coalescers can be compromised [8].        
In addition, FSII are toxic at the concentrations required for effective de-icing. ‘Water bottoms’ drained from storage tanks, fuel system sumps and filters inevitably contain higher concentration of di-EGME, creating concerns about the handling and disposal of these wastes [10]. FSII are only mildly irritating, but they are rapidly absorbed by the skin. Vapours can cause irritation to the eyes and the respiratory system. Long term effects include damage to the central nervous system, blood, skin, eyes, and kidneys [11]. Di-EGME is biodegradable in wastewater treatment system; however concentrations found in water bottoms could be high enough to disrupt the microbiological process. In the environment, the high oxygen demand required for decomposition results in less available oxygen for aquatic organisms [12].
Liquid hydrocarbons, such as aviation fuels present a number of problems for the selection of water scavenging compounds to be used as dehydrating icing inhibitors.
Aviation turbine fuel is a mixture of thousands of organic compounds; it is therefore critical that the scavenger of choice reacts exclusively with water, a notoriously weak nucleophile. The inventors have identified the alcohol-addition products of aldehydes and ketones (i.e., acetals, hemiacetals, ketals, hemiketals and ortho ester) as potentially possessing the appropriate levels of selectivity and stability for our purposes.
An important consideration is for the compound to be combustible as both scavenger and by-products must be readily combustible and leave no residue.
Because water is a relatively small molecule (molecular mass MM=18 g/mol), a relatively large number of reacting moles of scavenger—assuming a 1:1 reaction stoichiometry—are required to dehydrate the jet fuel. In the example mentioned above, 3.62 L of free water requires ca. 200 mol of scavenger. It is important therefore to ensure that the MM of a candidate scavenger is as low as possible, or possesses multiple reactive sites (higher reaction stoichiometry).
A selective scavenger which is switchable (i.e. on/off) can be stored without degradation (off), and yet be activated (on) when mixed with fuel. This is best achieved by choosing a process that is catalysed by mild acid, a particular consideration when one recognises that many liquid hydrocarbons such as Jet A-1 and some hydraulic fluids are mildly acidic.
The products of reacting one mole of the scavenger with water would ideally produce one equivalent of FSII for optimum fuel protection. Although in principle, it is possible to design an acyclic scavenger which generates up to three equivalents of FSII upon hydrolysis, this would impact upon atom economy. We require then a functional group which can be incorporated into a cyclic system which—post hydrolysis—affords an acyclic alcohol, thereby conserving atom economy.
A water scavenger ideally must be sufficiently hydrophobic to be soluble in, and therefore protect jet fuel—yet the product of the reaction with water must be sufficiently hydrophilic to preferentially partition into residual water to act as an effective FSII.
The use of acetals and ketals derived from sugar mannose for FSII has been explored by Mushrush and co-workers (U.S. Pat. No. 5,705,087). These compounds prove to be stable (up to 2 years in jet fuel) and effective as icing inhibitors, and importantly were found to be environmentally benign and relatively nontoxic at the necessary concentration [10, 13]. Additionally ketals have been used in fuels (U.S. Pat. No. 2,878,109).
Unlike an icing inhibitor, a water scavenger must undergo a chemical reaction at the very low operating temperatures (T) commonly encountered in an aircraft. Such low temperatures can dramatically reduce the kinetic energy of participating molecules, to the point that a chemical reaction slows down or effectively stops. It is necessary then to ensure that the energy barrier to reaction (Ea)—is small. This ensures a fast reaction even at low temperatures, which corresponds to a high velocity constant (k). The relationship between temperature, energy of activation and reaction velocity is expressed in the Arrhenius equation (I) [14].
                    k        =                  A          ⁢                                          ⁢                      e                                          -                Ea                            RT                                                          (        I        )            
By examining the rate constants for a range of candidate scavengers (i.e., acetals, ketals, and ortho esters) with water, the inventors found clear correlations between structure, and the barrier to reaction, Ea (FIG. 1). Hydration rates for acetals increase a thousand fold with the replacement of a single hydrogen atom at the functional carbon atom with an alkyl group (1→2 FIG. 1). A ten thousand fold rate increase accompanies progression to the related ketal (2→3 FIG. 1), and an extra 10 fold to the related ortho ester (3→4 FIG. 1). This important increase in relative rate is readily explained by a steric decompression factor as the tetrahedral intermediate collapses to the trigonal counterpart during the hydrolysis reaction, or by the fact that the rate increase corresponds with the stability of the corresponding dialkoxy carbenium intermediate (see later—Scheme 2) [15].
The fastest rate within the series of alcohol addition products of carbonyl compounds is achieved with the ortho ester 4 (FIG. 1), which is readily hydrolysed because there are three oxygen atoms with lone-pairs which may increase the basicity of the leaving group —ROH (see later—Scheme 2).
In conclusion, ortho esters appear to be the most promising candidate for a dual purpose additive meeting the criteria outlined above. The next step was to examine the feasibility of using the acid catalysed hydrolysis of a low molecular weight ortho ester to dehydrate jet fuel. FIG. 2 shows the complex nature of Jet fuel.